The respiratory system obtains oxygen from the air and transports it to cells via respiration. Oxygen diffuses into the lungs and blood, while carbon dioxide diffuses out of the blood and into the air. The blood then transports gases between the lungs and body tissues via internal respiration. Key components include the nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles and alveoli where gas exchange occurs through thin epithelial walls. Hemoglobin transports oxygen in the blood and facilitates diffusion in tissues through factors like pH and temperature changes.

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The Respiratory System:
“Respiration” is an everyday term that is often used to mean “breathing.”
Definition:
Respiration is a series of chemical reactions by which the living cell obtains energy for its various life
functions from various types of food
 Oxygen (O2) is used by the cells
 O2 is needed for conversion of glucose to cellular energy (ATP)
 Carbon dioxide (CO2) is produced as a waste product
 The body’s cells die if either the respiratory or cardiovascular system fails
Types of respiration
Two types of respiration
i) External Respiration
ii) Internal Respiration
I) External Respiration:
It is the direct exchange between the outer environment and respiratory surfaces (lungs, gills, skin etc)
ii) Internal Respiration:
It involves both gaseous exchange between the body fluid (blood) and tissue as well as the cellular
metabolism where Oxygen is utilized inside the cell.
As a result of cellular respiration energy is produced

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Cellular respiration is the aerobic breakdown of glucose in the mitochondria to make ATP.
Respiration Includes
I) Pulmonary ventilation
II) External respiration
iii) Transport of respiratory gases
iv) Internal respiration
i) Pulmonary ventilation
 Air moves in and out of lungs
 Continuous replacement of gases in alveoli (air sacs)
ii) External respiration
 Gas exchange between blood and air at alveoli
 O2 (oxygen) in air diffuses into blood
 CO2 (carbon dioxide) in blood diffuses into air
iii) Transport of respiratory gases
 Between the lungs and the cells of the body
 Performed by the cardiovascular system
 Blood is the transporting fluid
iv) Internal respiration
 Gas exchange in capillaries between blood and tissue cells
 O2 in blood diffuses into tissues
 CO2 waste in tissues diffuses into blood
Properties of Respiratory surfaces
- Large Surface area
- Moisturize surfaces
- Thin epithelium
- Ventilation
- Capillary network
Zones of Respiratory Organs
Respiratory organ consists of two zones
1) Conducting zone
2) Respiratory zone
1) Conducting zone:
 Respiratory passages that carry air to the site of gas exchange
 Filters, humidifies/moisturizes and warms air
2) Respiratory zone:
 Site of gaseous exchange
 Composed of
 Alveolar ducts
 Alveolar sacs

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Respiratory System
“Respiratory systems constitute those organs in animals that exchange gases with the environment.
Respiratory System of MAN:
Components:
Pathway of Inhaled and Exhaled Air:
1) Nasal cavity
2) Pharynx (Throat)
3) Larynx (Voice Box)
4) Trachea (Windpipe)
5) Bronchi
6) Bronchioles
7) Alveoli (Site of gas exchange)
1) Nose
 Provides airway for air passage
 Moistens and warms air (lined by mucus membrane of ciliated epithelium)
 Filters air (Hair and cilia)
 Resonating chamber for speech
 Olfactory receptors

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2) The Pharynx (throat)
 Length is about 4.5 inches
 Parts: three parts
i) Naso-pharynx
ii) Oro-pharynx and
iii) Laryngo-pharynx
 Houses tonsils (they respond to inhaled antigens)
 Uvula closes off nasopharynx during swallowing so food does not go into nose
 Epiglottis posterior to the tongue: keeps food out of airway
 Oropharynx and laryngopharynx serve as common passageway for food and air
 Lined with stratified squamous epithelium for protection
3) The Larynx (voice box):
 Extends from the level of the 4th
to the 6th
cervical vertebrae
 Attaches to hyoid bone superiorly
 Inferiorly is continuous with trachea (windpipe)
Functions: Perform three functions
1. Produces vocalizations (speech)
2. Provides an open airway (breathing)
3. Switching mechanism to route air and food into proper channels
 Closed during swallowing
 Open during breathing

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4) Trachea (the windpipe):
 About 5 inches in length and one inch in diameter
 Descends into mediastinum
 Divides in thorax into two main (primary) bronchi
 16-20 C-shaped rings of hyaline cartilage
joined by fibroelastic connective tissue
 Flexible for bending stays open despite
pressure changes during breathing
 Trachealis muscle can decrease diameter of trachea
 Inner lining of trachea is also lined with mucous epithelial ciliated cells – filters, warms and
moistens incoming air

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Trachea (Windpipe):
Rings of cartilage maintain shape of trachea, to prevent it from closing. Forks into two bronchi.
- Right main bronchus (more susceptible to aspiration)
- Left main bronchus
5) Bronchi (Sing. Bronchus):
Each bronchus leads into a lung and branches into smaller and smaller bronchioles, resembling an inverted
tree. Same cartilageous rings as in Trachea but is irregularly distributed cartilageous plates.
 Main (primary) bronchi divide into secondary bronchi, each supplies to one lobe of Lung
 3 on the right (as there are 3 lobes in right Lung)
 2 on the left (as there are 2 lobes in left Lung)
 Secondary (Lobar) bronchi branch into tertiary
(segmental bronchi)
 Continues dividing: about 23 times
6) Bronchioles:
Fine tubes (one mm or less in diameter) that allow passage of air.
Smallest (terminal) bronchioles are less than 0.5 mm diameter
Bronchioles totally lack cartilages.
Mainly composed of circular smooth muscle layer which constricts bronchioles.
Epithelium of bronchioles is covered with cilia and mucus.
 Mucus traps dust and other particles.
Ciliary Escalator: Cilia beat upwards and remove trapped particles from lower respiratory airways. Rate
about 1 to 3 cm per hour.
Branchioles continously divides and subdivides deep into the Lungs and finally open into a large numbers
of air sacs.

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Carina
Point where trachea branches (when alive and standing is at T7)
 Mucosa highly sensitive to irritants: cough reflex

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Respiratory Zone:
 End-point of respiratory tree
 Structures that contain air-exchange chambers are called alveoli
 Respiratory bronchioles lead into alveolar ducts
 Ducts lead into terminal clusters called alveolar sacs – are microscopic chambers
 There are 3 million alveoli!
Gas Exchange:
 Air filled alveoli account for most of the lung volume
 Very great area for gas exchange (1500 sq ft)
 Alveolar wall
 Single layer of squamous epithelial cells
 0.5um (15 X thinner than tissue paper)
 External wall covered by capillaries
 This “air-blood barrier” (the respiratory membrane) is where gas exchange occurs
 Oxygen diffuses from air in alveolus (singular of alveoli) to blood in capillary
 Carbon dioxide diffuses from the blood in the capillary into the air in the alveolus
In the alveolus
 The respiratory surface is
made up of the alveoli and
capillary walls.

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Surfactant:
Surfactant is a detergent-like substance which is secreted in fluid coating alveolar surfaces – it decreases
surface tension so prevent collapsing of alveoli
 Without it the walls would stick together during exhalation
 Reduces surface tension of the lung allowing the oxygen and carbon dioxide across the membrane.
 Premature babies – problem breathing is largely because lack surfactant
Lungs:
 Each is cone-shaped with anterior, lateral and posterior surfaces contacting ribs
 Superior tip is apex, just deep to clavicle
 Concave inferior surface resting on diaphragm is the base
Lungs and Pleura
Around each lung is a flattened sac of membrane called pleura
Parietal pleura – outer layer
Visceral pleura – directly on lung
Pleural cavity – slit-like potential space filled with
pleural fluid
 Lungs can slide but separation from pleura
is resisted (like film between 2 plates of glass)
 Lungs cling to thoracic wall and are forced
to expand and recoil as volume of thoracic cavity
changes during breathing

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Lobes of Lungs:
 Right lung: 3 lobes
 Upper lobe
 Middle lobe
 Lower lobe
 Left lung: 2 lobes
 Upper lobe
 Lower lobe
Ventilation/Breathing:
 Breathing = “pulmonary ventilation”
 Pulmonary means related to the lungs
 Two phases
 Inspiration (inhalation) – air in
 Expiration (exhalation) – air out
 Mechanical forces cause the movement of air
 Gases always flow from higher pressure to lower
 For air to enter the thorax, the pressure of the air in it has to be lower than atmospheric
pressure
 Making the volume of the thorax larger means the air inside it is under less pressure
(the air has more space for as many gas particles, therefore it is under less pressure)
 The diaphragm and intercostal muscles accomplish this
Muscles of Inspiration:
 During inspiration, the dome shaped diaphragm flattens as it contracts
 This increases the height of the thoracic cavity
 The external intercostal muscles contract to raise the ribs
 This increases the
circumference of the thoracic cavity

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Expiration:
 Quiet expiration in healthy people is chiefly passive
 Inspiratory muscles relax
 Rib cage drops under force of gravity
 Relaxing diaphragm moves superiorly (up)
 Elastic fibers in lung recoil
 Volumes of thorax and lungs decrease simultaneously,
increasing the pressure
 Air is forced out

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Moving air in and out:
 During inspiration (inhalation), the diaphragm and intercostal muscles contract.
 During exhalation, these muscles relax. The diaphragm domes upwards.
Interesting......
• At rest, the body takes in and breathes out about 10 litres of air each minute.
• The right lung is slightly larger than the left.
• The highest recorded "sneeze speed" is 165 km per hour.
• The capillaries in the lungs would extend 1,600 kilometres if placed end to end.
• We lose half a litre of water a day through breathing. This is the water vapour we see when
we breathe onto glass.
• A person at rest usually breathes between 12 and 15 times a minute.
• The breathing rate is faster in children and women than in men.
Transport of gases
i) Transport of Oxygen
ii) Transport of Carbon dioxide
Air entering the lungs contains more oxygen and less carbon dioxide than the blood that flows
in the pulmonary capillaries.

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 The resting body requires 250ml of O2 per minute.
 Four to six billion haemoglobin containing red blood cells are present in the human body
 The haemoglobin allows nearly 70 times more O2 than dissolved in plasma.
O2 is transported by the blood either
i) Combined with haemoglobin (Hb) in the red blood cells (98.5%) or
ii) Dissolved in the plasma (1.5%).

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Hemoglobin binds to oxygen that diffuses into the blood stream.
Hemoglobin Loading and Unloading of Oxygen
Hemoglobin is found in red blood cells
Functions:
i) Transports oxygen
ii) Transport carbon dioxide
iii) Helps buffer blood
As carbon dioxide is picked up from tissues it is converted into carbonic acid:
CO2 + H2O <-----> H2CO3 <----> H+
+ HCO3
-
Carbon Carbonic acid Carbonate ion
dioxide

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Hemoglobin picks up most H +
ions, so they do not acidify the blood.
Haemoglobin
Haemoglobin Saturation
 Haemoglobin saturation is the amount of oxygen bounded by each molecule of haemoglobin
 Each molecule of haemoglobin can carry four molecules of O2.
 When oxygen binds to haemoglobin, it forms OXYHAEMOGLOBIN;
 Haemoglobin that is not bound to oxygen is referred to as DEOXYHAEMOGLOBIN.
 The binding of O2 to haemoglobin depends on the PO2 in the blood and the bonding strength, or
affinity, between haemoglobin and oxygen.
The Oxygen Dissociation Curve (ODC)
ODC reveals the amount of haemoglobin saturation at different PO2 values.
The Oxygen Disassociation Curve
In the lungs the PO2 is approximately 100mm Hg at
this Partial Pressure haemoglobin has a high
affinity to O2 and is 98% saturated.
In the tissues of other organs a typical PO2 is 40
mmHg here haemoglobin has a lower affinity for
O2 and releases some but not all of its O2 to the
tissues. When haemoglobin leaves the tissues it is
still 75% saturated.

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Haemoglobin Saturation at High Values
Haemoglobin Saturation at Low Values

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Factors Altering Haemoglobin Saturation
Hemoglobin and Oxygen Transport
 Oxygen is transported by hemoglobin (98.5%) and is dissolved in plasma (1.5%)
 Oxygen-hemoglobin dissociation curve shows that hemoglobin is almost completely saturated
when P02 is 80 mm Hg or above. At lower partial pressures, the hemoglobin releases oxygen.
Factors Affecting Haemoglobin Saturation
 Blood temperature…
 Blood acidity…(pH of blood)
 Carbon Dioxide concentration
 Also heavy exercise......
Factors affecting Disassociation
BLOOD TEMPERATURE
 increased blood temperature reduces haemoglobin affinity for O2
 hence more O2 is delivered to warmed-up tissue
Factors Affecting Haemoglobin Saturation – Blood Acidity If the blood becomes more acidic the
dissociation curve shifts right.
This means that more oxygen is being uploaded from the haemoglobin at tissue level.
See overhead.
Factors Affecting Haemoglobin Saturation – Blood Acidity
The rightward shift of the curve is due to a decline in pH. This is referred to as the BOHR effect.
Factors Affecting Haemoglobin Saturation – Blood Acidity
The pH in the lungs is generally high.
So haemoglobin passing through the lungs has a strong affinity for oxygen, encouraging high saturation.

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At the tissue level, however the pH is lower, causing oxygen to dissociate from haemoglobin, thereby
supplying oxygen to the tissues.
• Factors Affecting Haemoglobin Saturation – Blood Acidity
• With exercise, the ability to upload oxygen to the muscles increases as the muscle ph decreases.
Factors Affecting Haemoglobin Saturation – Blood Temperature Increased blood temperature shifts the
dissociation curve to the right, indicating that oxygen is uploaded more efficiently. Factors Affecting
Haemoglobin Saturation – Blood Temperature Because of this, the haemoglobin will upload more oxygen
when blood circulates through the metabolically heated active muscles.
In the lungs, where the blood might be a bit cooler, haemoglobin’s affinity for oxygen is increased. This
encourages oxygen binding.
BLOOD PH
 lowering of blood pH (making blood more acidic)
 caused by presence of H+
ions from lactic acid or carbonic acid
 reduces affinity of Hb for O2
 and more O2 is delivered to acidic sites which are working harder
CARBON DIOXIDE CONCENTRATION
 the higher CO2 concentration in tissue
the less the affinity of Hb for O2
 so the harder the tissue is working, the more O2 is released
Key Point
 Increased temperature and hydrogen ion (H+
) (pH) concentration in exercising muscle affect the
oxygen dissociation curve, allowing more oxygen to be uploaded to supply the active muscles.
Oxygen Content (2)
CaO 2 = (1.34 x Hb x SaO2) + (0.003 x PaO2)
amount carried by Hb amount dissolved in plasma
CaO2 = (1.34 x 14 x 0.98) + (0.003 x 100)
CaO2 = 18.6 ml/dl
* at 100 % Saturation, 1 g of Hb binds 1.34 ml of Oxygen !
SaO2: means saturation of HB with oxygen, PaO2: means partial pressure of Oxygen, CaO 2 : carrying
capacity of Hb
Carbon dioxide transport
Carbon dioxide is transported from tissues toward lungs in several ways
i) As Carboxy-haemoglobin (23%) (HbNHCOOH)
ii) by other plasma proteins (7%)
iii) As Bicarbonate (HCO3) (70%)
CO2 + H2O <-----> H2CO3 <----> H+
+ HCO3
-
Carbonic acid anhydrase

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CO2 Transport and Cl-
Movement

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Summary:Changes in Partial Pressures
Hemoglobin that has released oxygen binds more readily to carbon dioxide than
hemoglobin that has oxygen bound to it (Haldane effect)
 In tissue capillaries, carbon dioxide combines with water inside RBCs to
form carbonic acid which dissociates to form bicarbonate ions and
hydrogen ions
Qari Sami ullah
(Msc.Zoology + Health Technologist)
Samihaseen8@yahoo.com